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Tuesday, 8 July 2008

When I got married, I was astounded to discover that my wife didn't realize that it is impossible to heat anything wet above the boiling temperature of water, which is usually no more than 212 degrees Fahrenheit (or 100 degrees Celsius, but since this story takes place in one of the United States, we'll use Fahrenheit. Or at least try to . . .). I eventually realised that this was nothing unusual; most cooks express surprise that food could overcook at a setting of only 200º, or that it so easily burns to a black crisp at a setting of 350º.

So for the benefit of miseducated cooks, let me explain how it works:

Temperature is a way of measuring the rate at which molecules knock into each other. The faster they vibrate, the higher the temperature that can be measured. Also, the more furiously they knock into each other, the faster some of them are knocked loose. As a part of this process, crystalline and amorphous solids break down into liquids, and eventually liquids break down into gasses. At a high enough temperature, the only possible state of matter is known as plasma, and all molecules are energised.

Adding heat to a substance has one of two effects on it: it either makes its molecules vibrate faster (raising the temperature), or it breaks down the molecular bonds holding the substance together (thereby melting a solid or evaporating a liquid).

At sea level, this means that adding heat to ice warms it up, but only to 32º; at that point, all heat transferred into ice converts it to 32º water (It shouldn't be too hard to convert the realisation that ice can never be any hotter than 32º to one that anything wet--hence, containing water--can never be any hotter than 212º). Once all the ice has become water, the water itself begins to warm up from 32º. The water continues to absorb heat as its temperature approaches about 212º (slightly more if it is in solution), at which point it stops getting hotter and begins converting to 212º steam. Once all the water has become steam, then the steam can begin to heat up again; 600º steam is common in high-pressure turbines.

To apply this principle to food, take the example of making maple syrup from sap. It takes a tremendous amount of heat to raise the temperature of the sap from freezing to around 218º, during which time the sap evaporates at a faster and faster rate. Turning up the heat makes it boil all the more furiously, but it won't raise the temperature of the sap any higher. Since the goal is to remove all water from the sap, this is good. Once the water is gone, however, the sap starts heating up again, and unless it is taken off the fire, will quickly climb past 250º, past 300º, past 350º, and soon burst into flame. This is not good. Thus one who watches his boiling pot carefully will know that the syrup is done by the time it reaches 218º. Better yet, a thermostatic cutoff can accomplish the same thing without the need for such vigilance.

If the cook, however, wants to retain any amount of moisture in the food being cooked, it is counterproductive to heat any portion of it above about 212º. Even in the best-case scenario, the center of the dish may be stewing along at only about 180º, while the outer edges boil off all their moisture and rapidly shoot up to a black-crust-forming 350º. All this can happen even with the thermostat set at a lower temperature. For a cook who has lots of time and would rather have overcooked food than burnt food, a setting of 200º to 250º is best. The other alternative is to use a pressure cooker, which keeps the moisture from boiling off until a considerably higher temperature is reached. Since the goal is hot food, not dehydrated food, this is the most effective option, with the happy benefit of also being the fastest one.

This, by the way, is why microwave ovens can cook food without burning it; they heat up the entire substance, not just the outer surface. Thus by the time the outer surface reaches 212º, the inner portion is already long done. It takes a lot of work to burn something in a microwave oven, unless the substance needs a temperature of 212º to be fully cooked--popcorn being a prime example. Popcorn will not pop until the moisture inside the kernel reaches 212º, which is why it is usually cooked in oil. No other food ever needs to be heated that high; even maple syrup can be safely (but more slowly) made at temperatures as low as 140º.

One more thing to note, is that a pot of water will start to boil at temperatures a lot lower than 212º, but only because the flames (or heating coils) below the pot are transferring their over-1000º-heat into the base of the pot. The inside base of the pot reaches 212º long before the water above it does, forming steam bubbles on its surface that gradually start to break loose and rise to the top faster than they can collapse. While the water itself may still be 180º, the bubbles forming on the bottom are around 212º (actually a little hotter, since the water pressure at that depth has some effect).

Now, to the myth of 200 degree waves, as reported by Laura Clout in The Telegraph. The lava entering the Pacific at the Kilauea volcano in Hawaii is around 2000º. It instantly gives up a lot of heat into the water washing over the face of the lava flow, quickly forming 212º water and even hotter steam. As the lava cools, it stops glowing red and turns dark, being constantly pushed farther into the water by the hotter lava behind it. The water, in turn, is constantly losing its heat to the vast ocean behind it, so the water temperature drops rapidly with distance from the lava.

Laura quotes surfer CJ Kahuna as saying:
"I was tentative to get too close at first, and for good reason, the boiling water there is well over 200 hundred degrees in some spots, super hot, and it quickly melted the wax on the surfboard."

While water will melt wax at temperatures considerably below 200º, its boiling point at sea level (which this obviously was) is always pretty close to 212º--even given the dissolved salt level of ocean water, not any higher than 213º. CJ is exaggerating a bit in claiming that the "boiling water" was "way over 200º in spots." It was, in fact (at the surface anyway), no more than 213º where it boiled away in contact with the lava, and progressively cooler (and not boiling) as its distance from the lava increased. Since CJ never got closer than 20 feet from the lava, it's safe to say he never encountered water even as hot as 200 degrees. Certainly the water that splashed over his feet was significantly cooler than that, or he would have been speaking from a hospital bed. And Laura was exaggerating even more in claiming that CJ was "surfing in boiling water."

Update April 1, 2009
This just in:
"Tentative at first, the adrenalin-junkie paddled to within 20ft of the lava, before moving back from water that reached 200C in places, melting the wax on his surfboard and burning his feet in the process."

Yah, 200ºC. Must be the day of the year, eh?

That's right. Our reporterette used a temperature setting DOUBLE that of boiling water, when she failed to recognise the use of Faherenheit in America.

1 comment:

I hope this article didn't come across disparagingly of CJ himself. That's his real name, by the way--almost. CJ apparently stands for Clement Junior. His full name is Clement Keliipoaimoki Kanuha III. He's a direct descendant of Hawaiian royalty, "the big kahuna."